Adjustable temperature variable output signal circuit

ABSTRACT

A mass fluid flow sensor is disclosed which utilizes a sensing bridge circuit to develop a sense (control) signal related to fluid flow. A fluid temperature variable resistor, separate from said bridge circuit, is utilized to implement temperature compensation so that a desired output signal is a function of sensed fluid flow, but is less dependent on fluid temperature than the sense (control) signal provided by the bridge circuit. A resistor in the bridge circuit is selected such that the sense (control) signal provided by the bridge circuit has a rate of change as a function of flow rate substantially independent of fluid temperature, but this sense signal still varies as a function of fluid temperature. This permits fluid temperature compensation of the bridge sense signal in a noncomplex and cost effective manner. An improved adjustable circuit (36-41) is provided for producing a desired temperature variable output signal (V os ).

This is a continuation, division, of application Ser. No. 094,953, filedSept. 9, 1987 now U.S. Pat. No. 4,854,167.

CROSS REFERENCE TO RELATED APPLICATION

The present invention is related to the invention described in copendingU.S. patent application Ser. No. 020,248, filed Feb. 27, 1987, entitled,"Mass Air flow Sensor", by Kevin Moran and Peter J. Shak, having thesame assignee as the present invention.

The present invention is related to an adjustable temperature variableoutput signal circuit. Previous circuits were difficult to adjust toachieve the desired DC bias level and the desired rate of temperaturevariation for an output signal. The present invention provides animproved circuit which can be readily adjusted to provide a desiredoutput signal.

SUMMARY OF THE INVENTION

An improved adjustable circuit for producing a desired temperaturevariable output signal is provided. This circuit is useable in the flowsensors described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, referenceshould be made to the drawings in which:

FIG. 1 is a schematic diagram of a mass fluid flow sensor embodying thepresent invention;

FIG. 2 is a combined cross section and schematic drawing illustrating afluid flow path and the location of the sensing elements of the presentinvention with respect to the fluid flow path;

FIG. 3 is a graph illustrating the relationship between fluid flow rateand fluid temperature caused changes in a control (sense) voltageprovided by the present invention and provided by other sensors notconstructed in accordance with the present invention; and

FIG. 4 is a graph illustrating fluid flow rate versus control (sense)voltage amplitude at different fluid temperatures, and illustrating thisrelationship for the present invention and for other sensors notutilizing the present invention.

Detailed Description of the Preferred Embodiments of the Invention

Referring to FIG. 1, a schematic diagram of a mass fluid flow ratesensor 10 embodying the present invention is illustrated. The sensor 10essentially comprises a hot film bridge-type mass fluid flow sensor inwhich the power dissipation of the hot film (comprising a heaterelement) is controlled so as to provide a predetermined amount of heattransfer between the hot film and a fluid (either gaseous or liquid) theflow of which is to be measured. However, it should be noted thatcertain aspects of the present invention are also applicable to constantpower or constant current fluid flow sensor circuits which rely on heattransfer from a constant power dissipation heater resistor to sensingelements located downstream of the heater resistor in the fluid.

The sensor 10 includes a low magnitude heater resistor 11 which has asubstantial positive temperature coefficient of resistance. The heaterresistor 11 is positioned in one arm of a four-arm bridge circuitgenerally indicated by the reference number 12 in FIG. 1. The bridgecircuit 12 includes, in another bridge arm separate from the bridge armcontaining heater resistor 11, a temperature variable resistor 13, alsohaving a substantial positive temperature coefficient of resistance. Theresistor 13 is connected in series with a nontemperature varying, withrespect to the temperature of the fluid being sensed, resistor 14. Thebridge circuit 12 also includes resistors 15 and 16 shown in FIG. 1 withresistors 13 through 15 forming a first series circuit path between abridge power supply terminal 17 of the bridge circuit, at which acontrollable power supply voltage is applied, and a reference terminal18 of the bridge circuit which is directly connected to a fixedreference potential comprising ground potential. The heater resistor 11and the resistor 16 are connected in series and form a second seriescircuit path between the terminals 17 and 18. Balance of the bridgecircuit is determined by comparing the voltage at a terminal 19 betweenthe heater resistor 11 and the resistor 16 with the voltage at aterminal 20 corresponding to a series connection point between theresistors 14 and 15.

The bridge balance terminals 19 and 20 are provided as inputs to adifferential amplifier 21 which provides an amplified difference outputat a terminal 22 which is provided as one input to another differentialamplifier 23. The output of the amplifier 23 comprises an analog controlsignal that is provided at a terminal 24 which is provided, via aresistor 25, as an input to a driver stage 26 shown dashed in FIG. 1.The driver stage 26, as shown in FIG. 1, comprises an NPN transistor 27and a PNP transistor 28 connected as shown between a battery supplyterminal 29, at which a battery voltage V_(BAT) is provided, and thebridge power supply terminal 17, at which a controllable analog bridgepower supply voltage V_(B) is provided. In addition, the bridge powersupply terminal 17 is also connected, via a voltage divider circuit 30as a second input to the differential amplifier 23.

The circuit elements 11 through 30, as described above, essentiallycomprise a bridge-type hot film mass fluid flow rate sensor in which themagnitude of bridge balance is sensed by the differential amplifier 21,and a control signal is provided at the terminal 24 which controls thevoltage at the terminal 17 and the current provided by the driver stage26. The operation of the above-described circuitry in providing a signalrelated to mass fluid flow can best be understood by referring to FIG. 2which illustrates the positioning of the heater resistor 11 andtemperature variable resistor 13 in the fluid, the flow of which is tobe sensed.

Referring to FIG. 2, a fluid flow path 55 for a fluid 56 is defined bywalls 50 and 51 across which a thin electrically insulating polyimidefilm 52 is positioned. The film 52 is positioned edgewise so that itprovides minimum opposition to the flow path 55. On the polyimide film52, the heater resistor 11 is implemented via a temperature-dependent,low-resistance, metallization film deposited on the substrate 52. Alsoon the polyimide film 52, the temperature varying resistor 13, which hasa much higher resistance than heater resistor 11, is deposited as a filmmetallization. Connections to the resistive films 11 and 13 are providedby conductor metallization paths 53 also provided on the film 52. Thepaths 53 connect film resistors 11 and 13 to additional sensorelectronics, generally designated by the reference number 54 in FIG. 2,also, preferably, mounted on an extension of the film 52. It isunderstood that the electronics 54 in FIG. 2 includes at least thecircuit elements 21 through 30 in FIG. 1.

The function of the sensor 10 is to provide a signal indicative of themass fluid flow rate of the fluid 56. Preferably, the sensor 10 willsense mass fluid flow in a predetermined direction corresponding to theflow path 55 in FIG. 2. The heater resistor 11 is positioned in thefluid 56, the flow of which is to be sensed, with the temperaturevarying resistor 13 also being positioned in the fluid 56, butpositioned upstream, with respect to the flow path 55, with respect tothe heater resistor 11. Preferably, the fluid flow to be sensedcomprises air flow into an internal combustion engine, such that themass fluid flow sensor 10 corresponds to a mass air flow sensor. Theoperation of the fluid flow sensor 10, as described above, will now bediscussed in connection with FIGS. 1 and 2.

Essentially, the driver stage 26 will provide some voltage at the bridgepower supply terminal 17 which will result in heater resistor 11producing a significant amount of power dissipation, in the form ofheat, some of which will be transferred to its surroundings, includingfluid 56. The polyimide film 52 is relatively thermally nonconductive,so that heat due to the power dissipation of the heater resistor 11 willprimarily be transferred to the fluid 56. The temperature varyingresistor 13 in the bridge 12 will dissipate only a very minimal amountof power and, therefore, not result in any substantial self-heating ofthis resistor. These relationships are due to the fact that the secondbridge circuit path comprising heater resistor 11 and resistor 16 has asubstantially lower resistance than the first bridge circuit pathcomprising resistors 13 through 15. It should be noted that thetemperature of the heater resistor 11 is intended to be maintained at apredetermined temperature above the temperature of the fluid 56(preferably 70° C. above fluid temperature) wherein this is due to thesubstantial power dissipation which will take place in the resistor 11due to self-heating. This is contrasted with resistor 13 which will beessentially maintained at the same temperature of the fluid 56 the flowrate of which is to be measured.

Essentially, the driver stage 26, in accordance with the control signalat the terminal 24, provides a voltage at the bridge power supplyterminal 17. This results in the temperature of the heater resistor 11being at a predetermined temperature above the temperature of the fluid56 whose flow rate is to be measured. When the flow rate of this fluidincreases, assuming that the fluid temperature upstream of the film 52remains constant, this will result in additional cooling of the heaterresistor 11 due to increased heat transfer between the fluid and theheater resistor 11. This, in turn, results in a lowering of theresistance of the resistor 11, and this results in a transientunbalancing of the bridge circuit 12. This bridge imbalance will tend toproduce a larger signal at the terminal 22 resulting in a larger controlvoltage at the terminal 24. This control voltage increase will result,via the driver stage 26, in providing a voltage increase at the bridgepower supply terminal 17, thereby resulting in increased current throughthe resistor 11. This current increase increases the temperature of theheater resistor and tends to restore the bridge circuit 12 to balance.Essentially, the control voltage at the terminal 24 will tend toimplement a servo control of the bridge circuit 12 to maintain thisbridge in a state of balance for any sensed fluid flow. This is achievedby controlling the voltage at the bridge power supply terminal 17 tocreate such a balanced state.

From the foregoing description it can be seen that both the controlvoltage at the terminal 24 and the bridge power supply voltage at theterminal 17 have analog amplitudes which vary in accordance with sensedmass fluid flow, and therefore these signals are indicative of sensedfluid flow. Temperature sensitive resistor 13 is provided in the bridgecircuit 12 to attempt to compensate for the effect of changes in fluidtemperature on the fluid flow signals provided at the terminals 24 and17. The type of system as described above is known and well understood,and the mechanical configuration discussed above with respect to FIG. 2is also known as per the above-referenced copending U.S. patentapplication Ser. No. 020,248, filed Feb. 27, 1987, and assigned to thesame assignee as the present invention.

While the function of flow sensors constructed in accordance with thepreceding description of FIGS. 1 and 2 is to provide an output massfluid flow rate signal which is independent of fluid temperature, thishas only been achieved with moderate success. The present inventionrecognizes that the fluid temperature compensation provided by theresistor 13 typically will not be sufficient for critical flow sensingapplications such as the sensing of mass air flow where the air is to bemixed with a fuel mixture for ignition in a vehicle internal combustionengine. In such an application, it is extremely important to have anaccurate flow sense signal which is independent of air temperature sinceair temperature can vary greatly. While resistor 13 does tend tominimize variations of the flow signal at the terminal 24 as a functionof fluid (air) temperature, measurements have shown that an appreciabledependency on fluid flow temperature still exists for sensorsconstructed as described above. Thus the present invention seeks toprovide additional correction and temperature compensation for suchcircuits, and this is accomplished in the following manner.

The voltage at the bridge power supply terminal 17 comprises an analogsignal which is related to and which varies substantially identically asthe control signal at the terminal 24. For the sensor 10 in FIG. 1, thesignal at terminal 17 is coupled to a correction circuit means, via avoltage divider 31, so as to provide an input to a differentialamplifier 32 which is part of the correction circuit means. The voltagedivider 31 and differential amplifier 32 essentially receive the signalat the terminal 17 and effectively modify it in accordance with areceived predetermined analog reference signal received at a terminal 33to provide a desired analog output signal at a terminal 34. Themagnitude of this output signal is related to both the signal at theterminal 17 and the reference signal at the terminal 33. The gain of thedifferential amplifier 32 is adjustable via an adjustable magnituderesistor 35 connected between the terminals 34 and 33.

A compensating temperature variable circuit element, comprising anadditional temperature sensitive resistor 36, which is separate from thebridge circuit 12 and control circuit elements 21-30, is positioned asshown in FIG. 2 in the fluid 56. The resistor 36 is positioned upstreamof the heater resistor 11 and also upstream of the temperature varyingresistor 13. The temperature variable resistor 36 has a substantialpositive temperature coefficient of resistance, and therefore itsresistance varies as a function of fluid temperature but issubstantially independent of the fluid flow rate. This is because thetemperature varying resistor 36, like the temperature varying resistor13, will exhibit minimal power dissipation such that changes in flowrate will have substantially no effect on the resistances of resistors13 and 36.

Referring again to FIG. 1, the temperature varying resistor 36 isconnected in series with a nontemperature varying, with respect to fluidtemperature, resistor 37 between an output terminal 38 of a differentialoperational amplifier 39 and an inverting input terminal 40 of thisamplifier. The terminal 40 is connected to ground through a resistor 41,and the amplifier output terminal 38 is connected through an isolationresistor 42 to the terminal 33. A noninverting (positive) input terminal40A of the differential amplifier 39 will receive a fixed referencevoltage. This fixed reference voltage is provided by a resistor 43connected between the battery voltage terminal 29 and a terminal 44. Avoltage reference semiconductor device 45 is connected between theterminal 44 and ground potential. A resistor divider network 46 isconnected, as shown in FIG. 1, between terminals 44 and 40A. The voltagereference semiconductor device 45 essentially corresponds to a precisionregulator having a substantially temperature independent output voltagecharacteristic. Such devices are readily available, such as Motorolasemiconductor device TL431. The operation of the components 31 through46 is as follows.

Essentially, the signal at the correction circuit means input terminal32A is just a scaled-down version of the signal at the terminal 17related to sensed mass fluid flow. As noted above, measurements haveindicated that typically this signal still has an undesired variation asa function of fluid temperature despite the fact that the fluidtemperature varying element 13 has been provided in the bridge circuit12. Thus the object of the present invention is to provide for a costeffective manner of compensating for this undesired temperaturevariation such that the desired output signal at the terminal 34 willstill be a function of the actual mass fluid flow rate, but will besubstantially less dependent on fluid temperature than the signal at theterminal 17. Essentially, the elements 36 through 46, in combinationwith selective adjustment of the magnitude of the resistance of resistor14 will produce this improvement in a very cost effective manner. Thisis accomplished as follows.

Referring to FIG. 3, a series of graphs are illustrated whichessentially plot voltage difference versus actual flow rate of the fluid56 between the walls 50 and 51. The horizontal axis in FIG. 3 is flowrate of the fluid 56, and the vertical axis is the voltage differencebetween the voltage V_(B) at the terminal 17 taken at an elevated fluidtemperature of 70° C. (degrees Centigrade), for example, and the voltageV_(B) at the terminal 17 taken at normal room temperature, such as 25°C. Clearly if the bridge temperature varying element 13 completelycompensated for all fluid temperature variations, the end result shownin FIG. 3 would comprise a line coincident with the horizontal axis.This would thus indicate that for any mass flow rate over an entirerange of fluid flow rates to be measured, such as between 6 and 360kg/hr (kilograms per hour), there would be a zero change in the voltageat the terminal 17 due to fluid temperature variation of 25° C. and 70°C. In such a situation, the present invention would not be needed sincethe bridge circuit 12 and circuitry 21-30 would already produce anoutput signal indicative of fluid flow rate and independent of fluidtemperature. However, measurements of circuits comprising the components11 through 30 have indicated that this is not readily achievable.

More specifically, adjustment of the magnitude of the bridge resistor 14was attempted so as to achieve a zero change in the voltage V_(B) at theterminal 17 for fluid temperatures of 25° C. and 70° C. for all flowsover the flow range of 6 to 360. The results are shown in FIG. 3.

Curve A in FIG. 3 illustrates that for a magnitude of resistor 14 of 160ohms, zero change, as function of temperature in the voltage V_(B) atterminal 17 is achieved only at a low flow rate of 6, whereas at higherflow rates there is a very large negative deviation of the voltage V_(B)at the terminal 17 at 70° C. as compared to the V_(B) voltage at 25° C.Curve B indicates that for resistor 14 having a value of 70 ohms, animproved situation is achieved, but again a zero change in the voltageat the terminal 17 as a function of temperature only occurs for only oneflow rate. Curve C indicates that for resistor 14 having a value of 0ohms, a zero change in the voltage at the terminal 17 is never achieved.FIG. 3 illustrates the curves A, B and C in terms of the change (delta)of the voltage V_(B) at the terminal 17 as a function of fluid flow forvarious values of the resistor 14 in the bridge circuit 12. This changein V_(B) represents the difference in the voltage V_(B) at the terminal17 taken at a fluid temperature of 70° C. versus the voltage V_(B) takenat a fluid temperature of 25° C.

FIG. 4 essentially illustrates the same relationships plotted as afunction of the amplitude of the voltage V_(B) at the terminal 17 as afunction of fluid flow rate. However, it should be noted that FIG. 4 isnot drawn to scale and just indicates general relationships.

In FIG. 4, for fluid at room temperature,

corresponding to 25° C., a curve 60 is generated which illustrates thatthe voltage V_(B) is a nonlinear function of sensed fluid flow rate overa flow range of 6 to 360. However, when curve 60 is compared with curvesA', B' and C' in FIG. 4, which correspond to curves A, B and C in FIG.3, respectively, and which curves correspond to the amplitude of thevoltage V_(B) at 70 degrees of fluid temperature over the same fluidflow rate range for different values of resistor 14, it is apparent thatcompensating for fluid temperature variations of the voltage V_(B) overthe entire fluid flow rate range would be extremely complex regardlessof which value of resistor 14 was selected so as to implement either ofthe curves A', B' or C'. Of course, curve B' appears to provide the bestcompromise situation, but still, as is apparent from FIGS. 3 and 4,compensation for these types of temperature variation would be complex.This is because the amount of needed fluid temperature compensation isalso a function of fluid flow rate.

The present invention proposes the selection of a resistance value forthe resistor 14 which, as shown by curve 61 in FIG. 3, results in asubstantially constant offset or change in the voltage V_(B) at 70degrees as opposed to 25 degrees throughout the entire flow range of 6to 360. This corresponds to a resistor 14 value of 35 ohms. The factthat a substantially constant change in V_(B) as a function oftemperature over the flow range of 6 to 360 is provided by the curve 61results in essentially implementing a series of parallel curves such ascurves 62 and 63 in FIG. 4, for any fluid temperature other than 25degrees C. Curve 62 in FIG. 4 represents the voltage V_(B) at 70° C. andcurve 63 in FIG. 4 represents the voltage V_(B) at 115° C.

It can be seen that the family of curves 60, 62 and 63 in FIG. 4essentially result in the rate of change of the voltage V_(B) atterminal 17, as a funCtion of flow rate over a "substantial range" (atleast one sixth, e.g., 6 to 60, of the total desired flow rate range6-360) of fluid flow rates to be measured, is substantially independentof fluid temperature, but that the actual amplitude of the signal V_(B)does vary as a function of fluid temperature. Preferably the preceding"substantial range" is at the low flow end of the total flow range so asto minimize the percent of error of the output signal, since at highflows more absolute error can occur without significantly altering thepercentage of error. The basic significance of selecting the magnitudeof the resistor 14 to achieve this characteristic is that nowcompensation for temperature variations of the control or sense voltageV_(B) at the terminal 17 can be much more readily and cost effectivelyachieved by the correction circuitry 31-46 of the present invention.This is partially because the actual variation (change) of the voltageV_(B) as a function of fluid temperature has now been made substantiallyindependent of fluid flow rate. Also, the variation of V_(B) as afunction of fluid temperature has been found to be, at least over thecritical low flow rate range of 6 to 60 kg/hr, substantially linear whenthe magnitude of the resistor 14 has been selected in accordance withthe present invention. Thus utilization of a temperature varyingresistor, such as the resistor 36, can now achieve the desired endresult since typically temperature varying resistors also varysubstantially linearly as a function of temperature. Even if a nonlinearfluid temperature variation of V_(B) was achieved, compensation wouldstill be substantially easier, since the amount of fluid temperaturecompensation would be independent of flow rate. Other temperaturesensitive elements, such as temperature varying diodes, could beutilized instead of the temperature varying resistor 36 as long as theresultant reference signal provided at the terminal 33 implemented thedesired substantially linear, fluid flow independent, temperaturevarying characteristic. The magnitude of the circuit components 36-46can be adjusted to achieve the amount of required temperature variationfor the compensation signal at terminal 33.

From FIG. 4 it can be seen that by selection of the proper resistancefor the resistor 14, the voltage V_(B) will essentially vary as anoncomplex and substantially linear function of fluid temperature forany flow rate in the range of 6 to 360. This is contrasted with curvesA', B' or C' which indicate, in FIG. 4, that if fluid temperature was,for example, 70° C., an extremely complex temperature compensation wouldbe required which would involve not only compensating for a fluidtemperature but also taking into account the fluid flow rate. Thepresent invention, by assuring that the voltage V_(B) will have its rateof change as a function of flow rate independent of fluid temperature,but the amplitude of the voltage V_(B) still being dependent on fluidtemperature, enables the present invention to readily implement atemperature correction of the signal V_(B). For the present inventiontemperature correction of V_(B), the desired output signal at theterminal 34 will essentially correspond to a single FIG. 4-type curverelating the amplitude of the signal at the terminal 34 to fluid flowrate regardless of the fluid temperature.

It should be noted that it may be possible to implement any of thecurves A', B' or C' for the bridge circuit 12 and then derive some sortof complex compensating signal to minimize or reduce temperaturevariation, but such compensation would be extremely complex unless theteachings of the present invention are followed. In addition, for bridgecircuits such as the circuit 12, typically no additional, separatetemperature compensation is implemented since it is generally presumedthat the temperature varying element 13 can sufficiently compensate forfluid temperature variations. As is shown by the present invention, thisis typically not the case.

The present invention first implements the bridge circuit 12, and thenimplements adjustment of the temperature compensating elements 36-41 toachieve the above-described desired results. The critical steps are (1)the selection of the magnitude of resistor 14 to achieve a change inV_(B), as a function of fluid temperature, which is substantiallyindependent of flow rate, and then (2) the adjustment of thecompensating temperature varying signal at the terminal 38. This isaccomplished as follows.

The proper value of resistor 14, so as to achieve the desired flatchange in V_(B) variation, for different fluid temperatures, as afunction of flow rate, is essentially learned by experimentation. Firstthe bridge circuit 12 and control components 21-28 are implemented asshown in FIG. 1. Then, since the heater resistor 11 is preferably to bemaintained at 70° C. above the fluid temperature, the value of heaterresistor 11 at 95° C. (which is 70° C. above normal ambient fluidtemperature of 25° C.) is measured. The value of resistor 13 at 25° C.is also measured. Then a fixed value for resistor 16 is selected, andvarious values of resistor 14 are tried with resistor 15 beingcalculated for each value of resistor 14 so as to maintain bridgebalance. Bridge balance for fluid temperature of 25° C. occurs when therelationship ##EQU1## is satisfied. The resistances of resistors 11-16are designated above by the notation R and corresponding numericalnotations. For each value of resistor 14, the change in V_(B), as afunction of flow, for fluid temperatures of 25° C. and 70° C. ismeasured. Thus an appropriate value of resistor 14 is selected to givethe desired substantially flat change in V_(B) , at least at low flowsof 6 to 60 kg/hr.

After the bridge circuit 12 has been set, adjustment of the components36-41 is needed to compensate for the V_(B) temperature variation. Firstthe actual temperature variation for fluid temperatures of 25° C. and70° C. of the signal at terminal 32A is measured. This signal is equalto kV_(B), with k being a proportionality constant determined by theresistor divider 31. Then the magnitude of the resistor 41 is adjustedso that the change between fluid temperatures of 25° C. and 70° C. ofthe signal at terminal 38 (V_(os)) matches the variation of the signalkV_(B) at terminal 32A. Thus the change in V_(os=the) change in kV_(B).Then the magnitude of the resistor 37 is adjusted and this adjusts theDC offset of the signal V_(os) at terminal 38 to match the DC level ofthe signal kV_(B) at terminal 32A. The adjustment of resistor 37 doesnot affect the previously-adjusted temperature variation of the signalV_(os) at the terminal 38, since the following relationship exists:##EQU2## where V_(R) is the fixed reference voltage at terminal 40A, andresistances of resistors are designated by R with correspondingnumerical notation. An adjustment of resistor divider 46 cannot providejust an offset adjustment of the signal V_(os), since any adjustment ofdivider 46 will change V_(R) and will cause a change in the temperaturevarying characteristic of the signal V_(os). The resistors 37 and 41could be adjusted by conventional resistor trimming techniques, such aslaser trimming. Thus the configuration of the components 36-41 providesa simplified circuit for implementing an adjustable temperature varyingsignal.

As stated above, the present invention contemplates providing an outputsignal at the terminal 34 which has a magnitude (amplitude) that variesas function of fluid flow rate but is substantially less dependent onfluid temperature than the sense (control) signals at the terminals 17and 24. However, even after fluid temperature compensation, a plot ofvoltage amplitude versus fluid flow rate for the signal at the terminal34 would still reveal that the amplitude of this output signal wouldvary nonlinearly as a function of sensed fluid flow rate. In order toprovide a linearized output, it is contemplated that the output terminal34 in FIG. 1 is connected as an input to a digital microprocessor 47which provides, at an output terminal 48, a linearized output. Thus theoutput at the terminal 48 comprises a signal having an amplitude whichvaries linearly as a function of sensed fluid flow rate, wherein thissignal, like the signal at the terminal 34, is substantially lessdependent on fluid temperature than the signals at the terminals 17 or24. The microprocessor 47 essentially comprises a digital table look-upcircuit to implement its stated linearization function.

While we have shown and described specific embodiments of thisinvention, further modifications and improvements will occur to thoseskilled in the art. Thus while preferably the present invention isdescribed in terms of a bridge-type sensor in which a control or sensesignal is developed to control power dissipation of a heater in thebridge, aspects of the present invention could be used for constantpower dissipation sensors in which a fluid flow sense signal, which doesnot implement a heater control function, is produced. All suchmodifications which retain the basic underlying principles disclosed andclaimed herein are within the scope of this invention.

We claim:
 1. A method for adjusting a circuit for producing a desiredtemperature varying output signal having a predetermined DC offset, thecircuit comprising:an amplifier having inverting and noninverting inputterminals and an output terminal at which said output signal, related tothe difference between signals at the input terminals, is provided; afixed, substantially nontemperature varying, nonzero reference signalprovided at one of the amplifier input terminals; a temperature variableresistor and a first nontemperature variable resistor connected inseries between another one of the amplifier input terminals and theamplifier output terminal; and a second nontemperature variable resistorconnected from said another one of said input terminals to a fixedreference potential, said first and second resistors being adjustable,the method comprising; first adjusting the magnitude of said secondnontemperature variable resistor to adjust the amount of temperaturevariation of said output signal; and then adjusting the magnitude ofsaid first nontemperature variable resistor to adjust the DC offset ofthe output signal without affecting the previously adjusted temperaturevariation of the output signal.